The present invention generally relates to a sputtering target, a method of producing the sputtering target, an optical recording medium having a recording layer formed by use of the sputtering target, and a method of forming the recording layer. More particularly, the present invention relates to a sputtering target and the production method thereof, the sputtering target being suitable for forming a recording layer through sputtering, and to a recording layer of an optical recording medium and the production method thereof, the recording layer being formed through sputtering by use of the sputtering target and comprising a phase-change optical memory material on which information is written and erased through a transition of two phases utilizing electromagnetic wave energy.
A phase-change optical memory material among various types of optical memory materials is known, and it is verified as being suitable for forming a recording layer or film of a rewritable optical disk. In the phase-change optical memory material, information is written, retrieved and erased through a transition of two phases of crystal structure conditions of the material, that is, from a crystal phase to a non-crystal or amorphous phase and vice versa, or from one crystal phase to another crystal phase and vice versa, and such a transition is caused by application of an electromagnetic wave such as laser light. Further, the phase-change optical memory material is suitable for forming a recording layer of an optical recording medium on which information stored therein is rewritable by use of a single beam.
Since it is difficult for a conventional magneto-optical memory to realize the rewritable feature mentioned above, much research and development on phase-change optical memory materials has been carried out. In addition, since the mechanism of an optical system for driving a phase-change optical memory material is simpler than the mechanism in the case of a magneto-optical memory, the phase-change optical memory materials have been found very useful.
U.S. Pat. No. 3,530,441 discloses typical examples of the phase-change optical memory materials which are chalcogen-structure alloy materials such as Ge--Te, Ge--Te--Sn, Ge--Te--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te, Se--As and the like.
There is another phase-change optical memory material which is a germanium-tellurium (Ge--Te) base alloy material to which gold (Au) is added as proposed in Japanese Laid-Open Patent Application No. 61-219692, for the purpose of achieving stability, high crystallization speed and the like. Similarly, to achieve stability, high crystallization speed and the like, a germanium-tellurium (Ge--Te) base alloy material to which tin (Sn) and gold (Au) is added is proposed in Japanese Laid-Open Patent Application No. 61-219692, and a germanium-tellurium (Ge--Te) base alloy material to which palladium (Pd) is added is proposed in Japanese Laid-Open Patent Application No. 61-270190.
In addition, in order to improve the writing/erasing repeatability characteristics, alloy materials which are Ge--Te--Se--Sb and Ge--Te--Sb having specified composition ratios have been proposed in Japanese Laid-Open Patent Application Nos. 62-73438 and 63-228433.
However, the conventional methods mentioned above do not completely satisfy the characteristics required for a phase-change optical recording medium on which information is rewritable. Among the problems which are still unresolved by conventional methods, the improvement of the write/erase sensitivity, the improvements of the overwrite repeatability and stability, and the improvement of the operational life of recorded and unrecorded portions of the material are significantly important. It is desirable to provide an optical recording medium having a recording layer of phase-change memory material in which the above mentioned improvements are realized.
It is known that, in order to deposit a recording layer or film of phase-change memory material on a substrate of an optical recording medium sputtering is performed using a sputtering target. However, in the prior art, there is no disclosure or teaching concerning the features of the sputtering target. That is, the prior art does not disclose or suggest what a sputtering target is suitable for forming a recording layer of phase-change memory material, which can realize the above mentioned improvements of the characteristics for the optical recording medium. Also, in the prior art, there is no disclosure or teaching how to form a recording layer through sputtering by use of the sputtering target, which can realize the above mentioned improvements of the characteristics.